Abstract

We use N-body simulations to investigate the structure of dark halos in
the standard cold dark matter cosmogony. Halos are excised from
simulations of cosmologically representative regions and are resimulated
individually at high resolution. We study objects with masses ranging
from those of dwarf galaxy halos to those of rich galaxy clusters. The
spherically averaged density profiles of all our halos can be fitted
over two decades in radius by scaling a simple "universal" profile. The
characteristic over- density of a halo, or equivalently its
concentration, correlates strongly with halo mass in a way that reflects
the mass dependence of the epoch of halo formation. Halo profiles are
approximately isothermal over a large range in radii but are
significantly shallower than r -2 near the center and steeper than
r-2 near the virial radius. Matching the observed rotation
curves of disk galaxies requires disk mass-to-light ratios to increase
systematically with luminosity. Further, it suggests that the halos of
bright galaxies depend only weakly on galaxy luminosity and have
circular velocities significantly lower than the disk rotation speed.
This may explain why luminosity and dynamics are uncorrelated in
observed samples of binary galaxies and of satellite/spiral systems. For
galaxy clusters, our halo models are consistent both with the presence
of giant arcs and with the observed structure of the intracluster
medium, and they suggest a simple explanation for the disparate
estimates of cluster core radii found by previous authors. Our results
also highlight two shortcomings of the CDM model. CDM halos are too
concentrated to be consistent with the halo parameters inferred for
dwarf irregulars, and the predicted abundance of galaxy halos is larger
than the observed abundance of galaxies. The first problem may imply
that the core structure of dwarf galaxies was altered by the galaxy
formation process, and the second problem may imply that galaxies failed
to form (or remain undetected) in many dark halos.

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